<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=\, initial-scale=1.0">
    <title>Document</title>
    <style>
        .no1 {
            width: 150px;
            height: 100px;
            background-color: rgb(96, 226, 128);
            border: 5px solid pink;

        }

        .no2 {
            width: 100px;
            height: 120px;
            margin: 100px;
            background-color: rgb(30, 34, 31);
            border: 5px solid pink;
            display: none;

        }

        .no1:hover .no2 {
            display: block;
        }
    </style>
</head>

<body>
    需求：当鼠标一放上去，就会出现另一个盒子
    <div class="no1">
        <div class="no2"></div>
    </div>

</body>

</html>